LTE (Long Term Evolution) uses OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and DFT-spread OFDM (Discrete Fourier Transform spread Orthogonal Frequency Division Multiplexing) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as seen in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, in that in each subframe (or transmission time interval, TTI) the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as control is illustrated in FIG. 3.
To transmit data in the uplink the mobile terminal has to have been first assigned an uplink resource for data transmission, on the Physical Uplink Shared Channel (PUSCH). In contrast to a data assignment in downlink, in uplink the assignment of resource blocks must always be consecutive in frequency, to retain the single carrier property of the uplink as illustrated in FIG. 4.
The LTE Rel-8 standard has recently been standardized, supporting bandwidths up to 20 MHz. However, in order to meet the upcoming IMT-Advanced requirements, 3GPP has initiated work on LTE-Advanced. One of the parts of LTE-Advanced is to support bandwidths larger than 20 MHz. One important requirement on LTE-Advanced is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE-Advanced carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a component carrier (CC). In particular for early LTE-Advanced deployments it can be expected that there will be a smaller number of LTE-Advanced-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE-Advanced carrier. The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that an LTE-Advanced terminal can receive multiple component carriers, where the component carriers have, or at least the possibility to have, the same structure as a Rel-8 carrier. Carrier aggregation is illustrated in FIG. 5.
The number of aggregated component carriers as well as the bandwidth of the individual component carrier may be different for uplink and downlink. A symmetric configuration refers to the case where the number of component carriers in downlink and uplink is the same, whereas an asymmetric configuration refers to the case that the number of component carriers is different. It is important to note that the number of component carriers configured in a cell may be different from the number of component carriers seen by a terminal: A terminal may, for example, support more downlink component carriers than uplink component carriers, even though the cell is configured with the same number of uplink and downlink component carriers.
Scheduling of the component carriers is done on the Physical Downlink Control Channel (PDCCH) via downlink assignments. Uplink grants are also signaled via PDCCH. Control information on the PDCCH is formatted as a Downlink Control Information (DCI) message. DCI messages for downlink assignments contain among others resource block assignment, modulation and coding scheme related parameters, hybrid-ARQ redundancy version, etc. In addition to those parameters that relate to the actual downlink transmission most DCI formats for downlink assignments also contain a bit field for Transmit Power Control (TPC) commands. These TPC commands are used to control the uplink power control behavior of the corresponding PUCCH that is used to transmit the hybrid-ARQ feedback.
The design of PDCCH in LTE Rel-10 follows very much that one in Rel-8/9. Assignments and grants of each component carrier are separately encoded and transmitted within a separate PDCCH. Main motivation for choosing separately encoded PDCCH over a jointly encoded PDCCH—here DCI messages from multiple component carriers would be lumped together into one entity, jointly encoded and transmitted in a single PDCCH—was simplicity.
In LTE Rel-10, the PDCCH is extended to include a Carrier Indicator Field (CIF), which is not present in LTE Rel-8/9. The CIF may consist of three bits attached to the DCI message which points to that component carrier the corresponding shared channel is located at. For a downlink assignment the CIF points to the component carrier carrying the PDSCH whereas for an uplink grant the three bits are used to address the component carrier conveying Physical Uplink Shared Channel (PUSCH). For simplicity this field is always three bits.
If CIF is configured, every downlink assignment and uplink grant contains the CIF bits, even if the assignment addresses PDSCH within the component carrier (or PUSCH within the linked uplink component carrier for uplink grants). With no CIF configured, the carrier aggregation looks like multiple parallel Rel-8/9 carriers, see FIG. 7. FIG. 8 shows the relation between PDCCH and PDSCH with configured CIF. A terminal configured with more uplink component carriers than downlink component carriers always requires an uplink grant with CIF.
The mapping of the CIF to component carriers could be done according to one of two different possibilities:                cell-specific mapping, i.e., the same mapping from CIF value to component carrier number is used by all user equipments (UEs) in the cell. The mapping could either be given according to rules or tables in the upcoming Rel-10 specifications or be signaled as part of the system information in the cell. With a cell-specific approach, the mapping is expected to be fixed or changed very infrequently.        UE-specific mapping, i.e. each user equipment (UE) has its own mapping from CIF to component carrier number. In this case, the CIF-to-component-carrier mapping is signaled as part of the UE-specific configuration information. Changing the mapping can, in this alternative, be more frequent than in the cell-specific alternative.        
Over time the user equipment will have the possibility to receive or transmit data on different component carriers, but not necessarily on all component carriers that a radio network node, such as an eNB, transmits in its cell(s). If the user equipment were required to receive all component carriers transmitted by the radio network node, this would result in short battery time and more memory consumption, for example. Furthermore, the radio network node has also the possibility to turn off component carriers, e.g., to enable power saving.
If UE-specific CIF-to-CC mapping is used, a problem will occur when the mapping from CIF-values to component carriers is updated. During updating of the mapping, the radio network node sends the reconfigured mappings to the user equipment and the network cannot communicate with the user equipment. This may lead to lost calls and degraded performance.